Audio amplifiers at present are normally provided with a partial turn-off (or standby) function controlled by a pin on the amplifier integrating device as shown by way of example in FIG. 1, which shows a standard audio amplifier 1 having a positive input connected via capacitor 2 to the IN signal pin; an inverting input connected to the amplifier output via capacitor 9 and feedback resistor 3, and grounded via resistor 4; and a supply pin connected to the V.sub.CC voltage supply line. A capacitor 14 is provided between the supply line and ground, and the amplifier output supplies a loudspeaker 5 d.c.-decoupled by capacitor 13. Amplifier 1 also presents a standby input connected to a parallel RC network including a capacitor 6 and a resistor 7 (normally external) connected between the standby input and ground. A switch 8 is series connected to resistor 7 and opened and closed externally for charging capacitor 6 to a value corresponding to normal operation of the amplifier, or discharging it via resistor 7 for activating the standby function.
The standby function, which may be user controlled or activated automatically in the event of a change in the signal source for eliminating undesired signals in the loudspeakers, consists in turning off practically the whole of the audio amplifier circuit, in particular the portion strictly relating to the output, so that the signal at the IN input is not transferred to the loudspeakers, and the amplifier (and on-off circuit) absorbs a very low current.
When switching from standby to on or off or vice versa, or from off to normal operation or vice versa, i.e., when the single-feed amplifier is turned on or off, the amplifier output undergoes considerable excursions (typically between 0 and 15 V in the case of a 30 V supply voltage) which are poorly controlled and comprise peaks in response to rapid transient states of the supply voltage or standby pin, which peaks are reproduced in the loudspeaker in the form of undesired noise (known in slang as "popping").
One attempt to eliminate the above noise has been to employ high-value capacitors for slowing down variations in the supply and standby voltages at the turn-on phase. Though valid, providing the charge time constant of the capacitors is low enough (slow charge), such a solution fails to provide for rapid operation of the audio system as a whole, as required for obvious reasons by the user. Moreover, as regards transient supply voltage, only charging of the capacitor is controllable, discharging being invariably rapid by virtue of the capacitor discharging into the low-value equivalent resistor of the overall circuit. As for the standby function, discharge is invariably rapid for the reasons explained below with reference to FIG. 2, which shows a simplified diagram of the turn-on circuit comprising capacitor 6, resistor 7 (having a low resistance R.sub.1) and switch 8, and in which the amplifier is represented solely by two resistors 10, 11 having the same resistance R, series connected between the supply and ground, and the midpoint of which presents standby voltage V.sub.ST.
When switch 8 in the FIG. 2 circuit is closed, capacitor 6 discharges into the total resistance of equivalent resistance R/2 parallel to resistance R.sub.1, i.e., into a resistance roughly equal to R.sub.1, thus setting the device to standby mode, whereas, when switch 8 is opened, capacitor 6 charges to the steady-state standby voltage via resistance R.sub.2 which is considerably greater than R.sub.1.
Consequently, whereas the capacitor may charge slowly, it discharges at a faster rate which cannot be slowed down on account of resistance R.sub.1, which must necessarily be low to ensure that, in standby mode, voltage V.sub.ST is low enough to keep the amplifier practically off. On the other hand, any increase in the capacitance of the capacitor would result in the amplifier being turned on again too slowly.